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Diffuse reflection and transmission of light, and the cosine law

When light encounters a surface with an unevenness comparable to or slightly greater than the wavelength of the light, there is no longer a single reflected or refracted ray. Instead, the light energy is dispersed in all directions from the point of incidence, as in scattering. Light that returns to the medium in which the incident light originates is called diffusely reflected light, while light that transmits into a second medium is called diffusely transmitted light. Generally speaking, the precise angular distribution of reflected and transmitted light depends on the angle of incidence of the light on the surface and the nature of the surface roughness. For surfaces with extremely fine particles, reflection is almost specular when the angle of incidence approaches 90°.

For computational convenience, the concept of a uniform diffuser is often used. A uniform diffuser is one whose reflected light distribution is independent of the angle of incidence; in directions at an angle of 0 with the surface normal, the reflected light intensity is proportional to cosine 0. This cosine law also applies to uniform diffuse transmission. However, no real surface fully meets the requirements of a uniform diffuser, although some surfaces, such as magnesium oxide powder coatings, closely approximate uniform diffusers.

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